There are growing signs that something truly horrific is about to happen in Hong Kong, and President Trump has essentially given China’s rulers a green light to crush the pro-democracy protests.

I think the role of the US president and other foreign leaders in this is more than performative. These protests are perceived as a threat to the authority of the mainland government (the protesters' demands include true representative democracy in Hong Kong and for Beijing to respect their autonomy; protesters have also defaced symbols of Beijing's authority and some have even called for independence) and the worrying precedent for how the Chinese government might respond is the 1989 massacre that broke up the Tiananmen Square protests.

In deciding whether or not to send in soldiers to quell the protests (i.e. to massacre civilians) the Chinese government will consider not only the fallout in Hong Kong and the mainland but they'll consider the international response as well. No one will go to war with China over this but economic sanctions and the use of the Magnitsky Act to target individuals could be harmful or even destabilizing. With Beijing recently hinting at their willingness to deploy troops it speaks volumes for the US president to use the same language as Beijing to characterize the protests ("riots") and to essentially say he considers the whole thing, use of troops included, as an internal matter of China. With carte blanche from Trump to do as they wish what other nation would want to stand up to China alone without American support?

The United States also breaks up protests and demonstrations that it doesn't like and that threaten state power, they have no moral authority to criticize another government for doing the same.

I strongly disagree with this for a couple of reasons. One is that there have been no mass protests in modern US history that threaten state power in the same way as these protests in Hong Kong do. The largest protests in recent American history (anti-Trump, civil rights, anti- Iraq or Vietnam wars) have been about changing policy or changing administrations but they don't threaten the state itself. To find a large scale popular movement that threatened the American state you'd have to go back to the Civil War. (Arguably you might also count certain strikes in the early 20th century but I don't know much about that.)

In China, though, the (only) ruling party is the state and the leader of the party is the leader of the country. If you are against Xi Jinping or the Chinese Communist Party you are threatening the state; conversely, you can be against Trump or the Republicans without being against the federal government itself. In the specific case of Hong Kong these protests are pushing back against the erosion of Hong Kong's autonomy and demanding that the democratic will of the people be represented, both ideas which greatly challenge the authority of the central government in a way that American protests against certain policies don't.

The second point (to bring things back to the original quote) is that with regard to "break[ing] up protests" there's no equivalency between what might happen in Hong Kong (Tiananmen 2.0) and what the US has done to their own protests in living memory. Closer parallels could be drawn between what's happened so far in Hong Kong and the worst actions of the US against their protesters but how close those parallels are is a different topic than the one at hand (i.e. the US response to Beijing's apparent threat to use overwhelming military force against the protesters).

As for arguments about whether or not the US has "moral authority" to criticize another government, that's a pretty big topic, so I'll limit myself to saying that we shouldn't let accusations of hypocrisy or ulterior motives be excuses to ignore basic human rights or to justify the actions of states who would abuse those rights. It would not be okay to kill thousands of innocent people for the 'crime' of standing up for their rights (NB: I'm definitely not saying the comment I'm replying to is advocating for or justifying that) and it would not be wrong for the US president or other world leaders to condemn the possibility of such actions.